The present invention relates to high voltage flexible transmission lines and, more particularly, to the class of transmission lines referred to in the industry as gas insulated transmission lines or by the initials GITL.
For many years the United States Department of Energy and the Electric Power Research Institute, Inc. (EPRI) have been sponsoring research to develop high voltage transmission system cables. One type of cable has a rigid metal sheath and employs an electronegative gas under pressure as the primary insulation. These cables are now generally designated, as noted above, by the initials GITL. Gases such as SF.sub.6 (sulfur hexafluoride) are used to increase the high voltage breakdown levels of these GITL cables. Research has been going on for at least ten years. A paper entitled "Installation and Field Testing of 138 kv SF.sub.6 Gas Insulated Station and Transmission Line" by T. F. Garrity, R. Matulic and G. Rhodes, was presented at the IEEE PES winter meeting in New York on Jan. 1-31, 1975. This paper contains, inter alia, six references to additional articles concerning the subject.
Rigid lines pressurized with gas as insulation have been produced and used, but because of the rigid character they must be prefabricated in relatively short lengths, generally no more than 60 feet, on a custom basis and have been used primarily in power stations. These cables employ either dielectric disc or post-type insulator supports for aligning the inner conductor element in appropriate relationship to an outer metallic tube element usually referred to as the sheath. The conductor element which is the current carrying member of the cable, as well as the sheath member, are usually fabricated from copper or aluminum. The conductor and sheath are held in concentric alignment by means of the insulating dielectric spacers which may be molded or vacuum cast from an organic or inorganic material such as an alumina-filled epoxy resin. Upon installation the space between the sheath and the conductor is then filled with a compressed gas such as SF.sub.6 which under operating conditions may be at a typical pressure of 50 p.s.i.g. at temperatures up to 150.degree. C. The problems encountered with such cable systems are considerable.
For example, in order to provide the necessary dielectric strength to withstand the high voltages, the rigid line sections, prior to operation, are normally purged or cleaned in an attempt to remove microscopic particles of metal which remain in the cable after construction is completed. However, complete elimination of the metal particles is impossible and during cable operation the metal particles remaining in the cable tend to migrate or oscillate between the conductor and sheath due to the highly stressed electric and magnetic fields. Movement of the remaining particles between the conductor and sheath can cause voltage breakdown. Prior art GITL systems have employed particle traps and other additional components in an effort to capture these particles before damage is done.
A major problem encountered in attempting to design transmission lines of the foregoing type for long runs when the line is intended to carry high currents is that resulting from differential thermal expansion. Because of the high voltages the conductor and shield must have significantly different diameters in order to provide an adequate gap therebetween, and when this gap is filled with an electrically insulating material, whether it be fluid or solid, such material also introduces thermal insulation. Consequently, a significant thermal gradient develops between the conductor and the sheath. The conductor tends to want to elongate as the temperature rises, and being at a higher temperature than the sheath, expands longitudinally relative thereto. However, if the ends of the cable are restrained at the respective terminations, the strain developed in the conductor must be accommodated in some manner and this produces extreme mechanical forces as the conductor tends to distort within the confines of the sheath. This can result in destruction of the spacers and/or failure of terminal connectors.
In my U.S. Pat. No. 2,998,472 issued Aug. 29, 1961 for "Insulated Electrical Conductor and Method of Manufacture" I disclosed a coaxial cable construction with two or more conductors intended primarily for radio frequency service. The cable contained an array of insulating tubes laid about a conductor and pressed into contact with the conductor to form a symmetrical array wherein the insulating cross-section provided a maximal amount of space and a minimal amount of dielectric mass. To construct the cable the insulating tubing was assembled in a loose array about the conductor. The insulating tubing and conductor or conductors were then drawn into a uniform diameter jacket of metal, organic material or other semi-rigid material which was thereafter drawn or otherwise reduced in size to cause the insulating tubes and conductors to be tightly packed into an array of the desired configuration. As a result the insulating tubing and the conductors were immovably secured against lateral movement in a predetermined configuration within the jacket, namely, with the conductor centered within the jacket.
It was suggested in my said patent that the jacket might be a wound armoured type such as that utilized in the familiar "BX cable", or it might take the form of round wire or flat wire braiding. However, in all instances the cables were intended for communication service, it being contemplated that they would generally be utilized for the transmission of audio frequency or radio frequency signals. Such service, particularly where both the inner and outer conductive elements were carrying current, did not produce severe thermal problems. Under such circumstance the frictional engagement between conductor, insulator tubes and jacket was adequate to restrict relative longitudinal movement to a minimum.
Another United States patent dealing with radio frequency cables was issued Feb. 11, 1964 under U.S. Pat. No. 3,121,136 to Mildner, entitled "Co-Axial Cable Having Inner and Outer Conductors Corrugated Helically in Opposite Directions". According to said patent "there is provided an air-spaced co-axial cable comprising an inner tubular corrugated conductor, an outer tubular corrugated conductor, at least one of the conductors being helically corrugated, and insulating material affording air spaces extending between the conductors longitudinally of the cable, the insulation material engaging against the crests of the corrugations on the inner conductor and against the troughs of the corrugations on the outer conductor."
While thermal problems must be considered when constructing coaxial cables for communication service, such as the cables contemplated by both my above-mentioned prior patent and the Mildner patent, such thermal problems are comparatively insignificant when compared with the thermal problems encountered in high voltage power transmission lines. Seeking maximum efficiency, electric power is transmitted at high voltage so as to reduce the current for a given power quantity. Nevertheless, power transmission involves very large currents which, within practical limits of conductor "copper", produce considerable heat. This heat is generated within the conductor. Since the conductor is separated by insulation from the outer sheath, and since such insulation not only insulates against electric current but also against caloric transmission, a considerable thermal gradient develops radially in the power cable. Naturally, this creates serious physical problems due to differing coefficients of thermal expansion characterizing the various materials making up a given cable. The problems arising from differential thermal expansion are exacerbated when attempts are made to produce long flexible lengths of cable suitable for high voltage power transmission service.